Self-expanding stents are useful for a variety of procedures requiring the maintenance of the patency of a bodily pathway. Such stents are generally biased to expand, such that when deployed, they assume an open position, pushing outward and into the surrounding area into which deployed. The radial expansion creates or maintains a pathway in a once occluded or weak area.
Avoiding the use of a sheath for deploying a stent, such as a self-expanding stent, is desired to avoid many of the shortcomings resulting from sheath deployment. For example, the sheath release delivery devices are difficult to reposition or remove and slow to operate. The stent may only be partially deployed prior to reconstrainment of the stent by the sheath in order to still reposition or remove the stent. Once the stent is fully deployed, i.e. radially expanded, the sheath cannot reconstrain the stent to allow it to be repositioned or removed. For example, utilizing a conventional outer sheath/inner catheter delivery device may cause the physician to inadvertently use excessive force and pull back the outer sheath too far, thereby prematurely deploying the stent in an incorrect position within a bodily lumen. At this step in the procedure, repositioning of the stent becomes difficult, if not impossible, because the stent has already radially self-expanded into the bodily lumen. Additionally, retraction of the outer sheath in a controlled manner is often difficult which may lead to uneven or inadvertent jerking back of the outer sheath and improper positioning of the stent.
Moreover, in a typical sheath release device where the outer sheath is proximally withdrawn, the first portion of the self-expanding stent to make contact with the body vessel is the most distal portion of the stent. This type of release may cause difficulty in accurately placing the proximal portion of the stent because the proximal portion of the stent may elongate or foreshorten while still covered by the outer sheath or after the sheath releases the stent. Thus, the positioning of the stent body in the central portion of the target region may be difficult with a distal stent release system. An additional drawback occurs with the sheathed stent delivery system where direct visualization of the stent is required. For example, in endoscopically placed stents, the sheath tends to prevent or obscure the location of the stent, making accurate placement of the stent more difficult. Accurate placement of the proximal portion of the stent and/or the stent body may be important in certain applications, for example to prevent stent migration or to properly open a stricture along the entire length of the stricture.
Further potential drawbacks for the conventional sheathed stent delivery system involve the stent placement within the system prior to use within a patient. Loading and anchoring of a conventional sheathed stent delivery device is an involved process that may require preloading the stent into the device so that the stent remains compressed within the sheath during shipment and storage prior to use in the patient. Extended compression of the stent may lead to an alteration in the stent mechanical properties.
Conventional sheathed stent delivery devices also require a high force to overcome the friction between the stent and the sheath that may also be a problem for proper stent placement within the patient. The introducer must be mechanically stronger to overcome the frictional forces to avoid undesirable frictional consequences such as stretching of the introducer catheters and hysterics in the movement of the stent. The sheathed stent delivery device also requires more space within an endoscope compared to a sheathless device and also adds additional expense to the delivery system.
Double-helical self-expanding wire stents that are able to assume a compressed state (i.e., a state wherein the diameter of the stent is less than when the stent is expanded) without the use of a sheath have been developed. For example, referring to FIGS. 1A-1C, self-expanding stents have been developed that may be constrained by rotating the ends of the stent in opposite directions. However, these stents may not compress or deploy in a uniform, predictable manner. Thus, while avoiding the shortcomings from using a sheath, these types of stents may present different drawbacks. For example, when in a compressed, un-expanded state, the stent's outer surface is not uniform throughout, and generally, a ridge on the outer surface of the stent is created during compression. When expanding, such stents may pop open and radially expand in a jumpy, non-uniform manner. The stent's non-uniform compression and expansion make deployment and repositioning the stent difficult due to the unpredictability of the stent's physical behavior.